This removes PKG_VERSION and PKG_RELEASE from most Makefiles, as the
value was never useful for Gluon packages; instead, PKG_VERSION is set
to 1 in gluon.mk.

It also removes two other weird definitions:

- gluon-iptables-clamp-mss-to-pmtu replicating the old PKG_VERSION logic
  from gluon-core, but without the fixed PKG_BUILD_DIR to prevent
  unnessary rebuilds
- gluon-hoodselector set GLUON_VERSION=3

- Move site check for prefix4 and extra_prefixes6 to gluon-core, so the
  rules don't need to be duplicated in several packages. This also fixes
  gluon-respondd not checking extra_prefixes6 at all when
  gluon-ebtables-source-filter is not installed as well.
- A redundant check for prefix6 is removed from gluon-l3roamd (this was
  already checked by gluon-core)
- A separate check for prefix4 remains in gluon-client-bridge, as the
  setting in mandatory there

In addition to significant internal differences in check_site_lib.lua (in
particular unifying error handling to a single place for the upcoming
multi-domain support), this changes the way fields are addressed in site
check scripts: rather than providing a string like 'next_node.ip6', the
path is passed as an array {'next_node', 'ip6'}.

Other changes in site check scripts:
* need_array and need_table now pass the full path to the sub fields to the
subcheck instead of the key and value
* Any check referring to a field inside a table implies that all higher
levels must be tables if they exist: a check for {'next_node', 'ip6'} adds
an implicit (optional) check for {'next_node'}, which allows to remove many
explicit checks for such tables

[Matthias schiffer: rebase, add a few more restrictions]

Some files have received some additional refactoring.

These packages don't work or don't make sense without batman-adv.